1. Field of the Invention
The present invention relates to a telecommunication system and a transmission method, and in particular to the ones preferably applicable to a mobile telecommunication system based on Code Division Multiple Access (CDMA) system.
2. Description of the Related Arts
A diversity handover (DHO) between base stations (BTS) used for a Third Generation Partnership Project (3GPP) system is accomplished by a selective synthesis/duplicate distribution at a mobile terminal (i.e., a user end (UE))/base station control apparatus (i.e., a radio network controller (RNC)). That is, the same data is transmitted and received by way of two or more radio transmission paths so as to select the data with good transmission quality (i.e., in which error-free data has arrived) is selected.
FIG. 1 is a diagram showing an image of an inter-base station DHO.
A mobile terminal UE is in telecommunication (sometimes abbreviated as “telecom” hereinafter) with a plurality of base stations Node B by way of a plurality of wireless lines Uu, and these base stations Node B is in telecom with a base station control apparatus RNC by way of a telecom line Iub. When carrying out a DHO, the same data is transmitted from the base station control apparatus RNC to individual base stations Node B, and the respective base stations Node B sends the same data to one mobile terminal UE. The mobile terminal UE applies a despreading process to each of the individual pieces of the same data sent from the respective base stations Node B, followed by synthesizing it, obtaining demodulation data, applying an error correction decoding process and reproducing transmission data. When a mobile terminal UE moves from under a management of a certain base station Node B to under a management of another base station Node B, a selective reception of data from a line with a good transmission quality results in selecting data from an approaching base station Node B, while naturally not selecting data from a leaving base station Node B, thereby accomplishing an interrupt-free handover.
When carrying out a DHO in the 3GPP system, however, a wireless capacity decreases with increased transmission paths because the same data is transmitted to a plurality of transmission paths.
FIG. 2 is a diagram showing a flow of data at carrying out a DHO in the 3GPP system in the case of the number of wireless transmission paths being two.
An original one piece of Information Data is copied and allocated to two wireless transmission paths. A cyclic redundancy check (CRC) bit is added to the Information Data, the resultant is turbo-coded, and a Terminal bit that is a redundancy bit is added to the turbo-coded. Then, a first interleaving process is applied, and the interleaved data is divided into two pieces of data for example. Then, a rate matching is applied, a second interleaving process is applied, divided into slots and sent out on a wireless frame.
As described above, since the information data is copied and transmitted from the base station control apparatus to base station, the line between the base station and base station control apparatus is extraneously used, and also the wireless line between the base station and mobile terminal extraneously used.
Furthermore, although individual wireless transmission paths are mutually separated by different spreading codes, an increase in power occurs for securing a desired signal to interference ratio (SIR) because another wireless transmission path becomes a noise for a certain wireless transmission path when performing despreading.
FIG. 3 is a diagram describing a degradation of a signal to interference ratio (SIR).
In the CDMA telecom system, data is transmitted by using a single carrier, and therefore data in a spread state in different wireless transmission paths are transmitted as being overlapped with one another in the same frequency band. A mobile terminal obtains a signal in a single wireless transmission path as a result of despreading, while data in another wireless transmission path is received in a state of being spread, thus becoming a noise for the despread data of a wireless transmission path. Therefore, such a noise increases with the number of wireless transmission paths.
A reference patent document 1 has disclosed handoff means allowing no instantaneous interrupt, having a cell diversity effect, and preventing a downlink wireless resource usage volume at a wireless base station from varying between the time of non-handoff and that of a handoff.
Patent document 1: Laid-Open Japanese Patent Application Publication No. 2000-217139